The present invention relates to a synthetic resin composition for a resin magnet, which is produced by mixing and dispersing a magnetic powder in a resin binder, and which is suitably used as a molding material for forming a molded resin magnet adapted for a magnet roller used for an electrophotographic system or an electrostatic recording system, and also relates to a resin magnet molded from the synthetic resin composition into a desired shape.
A development method characterized by supplying toner with the aid of a so-called jumping phenomenon has been used for an electrophotographic system or electrostatic recording system such as a copying machine or printer. To be more specific, according to such a development method, as a developing roller for visualizing an electrostatic latent image on a latent image support such as a photosensitive drum, a magnet roller formed of a resin magnet is disposed in a rotating sleeve, whereby toner is supplied to the surface of the latent image support with the aid of the jumping phenomenon, which allows the toner supported on the surface of the sleeve to be jumped on the latent image support by the magnetic force characteristic of the magnet roller, to visualize the electrostatic latent image.
The above-described magnet roller has been produced in accordance with the process of preparing a synthetic resin composition for a resin magnet by mixing a magnetic powder in a binder composed of a thermoplastic resin, and injection-molding or extrusion-molding the resin composition by using a mold around which a magnetic field is formed, thereby forming the resin composition into a roller shape and simultaneously magnetizing it into a desired magnetic characteristic.
With the recent advance of electrophotographic systems and the like, the above magnet roller has been required to have a more complicated magnetic force pattern, and to meet such a requirement, there have been often adopted a magnet roller including a plurality of magnet pieces. The magnetic pieces, each of which is molded from the above synthetic resin composition and magnetized into one magnetic pole as one component of the desired magnetic pattern, are stuck around a shaft, to thereby achieve the desired magnetic force pattern as a whole.
The synthetic resin composition for a resin magnet adapted for the above magnet roller has been produced by mixing and dispersing a magnetic powder of ferrite or a rare earth magnet alloy in a resin binder composed of a polyamide resin such as polyamide 6 or polyamide 12, or polypropylene.
However, with the recent tendency toward higher quality, higher speed, and higher accuracy of OA equipment, the above magnet roller has been required to have a higher magnetic force. Additionally, in other fields, resin magnets have been usually required to have higher magnetic forces.
To meet such a requirement, it is required to increase the content of a magnetic powder in a synthetic resin composition for a resin magnet adapted for a magnet roller and the like; however, if the content of a magnetic powder is increased, the flowability of the molten resin composition Is lowered, so that the moldability of the molten resin composition is significantly reduced, to cause both the variation in magnetic force of a molded product and degradation of the dimensional accuracy thereof. For this reason, the content of a magnetic powder in a synthetic resin composition has been limited. In other words, it has been difficult to add, to a resin composition, a magnetic powder in an amount large enough to obtain a necessary high magnetic force.
A resin magnet has been used not only for the above-described magnet roller but also for various general-purpose parts, particularly, for precise parts of electric/electronic equipment. In this case, such a resin magnet has been produced by molding the above resin composition into a desired shape. The resin magnet thus produced, however, fails to sufficiently satisfy high dimensional accuracies required for the magnet roller and precise parts of electric/electronic equipment.
To be more specific, a resin magnet used for a magnet roller of an electrophotographic system or electrostatic recording system and precise parts of electric/electronic equipment requires a very severe quality in terms of dimensional accuracy and environmental change in dimension; however, the resin magnet produced by molding the above resin composition is insufficient in its dimensional accuracy because of contraction of the resin after molding, and even if the resin magnet exhibits a good dimensional accuracy at the time of molding, it cannot keep the dimensional accuracy for a long-period of time because of expansion and/or camber of the resin magnet caused by absorption of moisture in a service environment.
By the way, a resin magnet for a magnet roller has been produced in accordance with a process of preparing a resin composition by mixing and dispersing a magnetic powder in a resin binder and extruding the mixture into pellets, and injection-molding or extrusion-molding the pellets of resin composition into a roller shape and simultaneously magnetizing the roller-shaped product into a desired magnetic characteristic.
The above-described pellets of the resin composition to be injection-molded or extrusion-molded into a resin magnet have been produced by mixing a magnetic powder in a binder resin and extruding the mixture into pellets. In this case, as the above binder resin, a resin generally commercially available in the form of pellets has been used. The pellets of resin composition have been produced by pulverizing the pellets of resin, mixing the pellets thus pulverized with a magnetic powder, and extruding the mixture into pellets.
The reason why the pellets of resin are pulverized once before being mixed with a magnetic powder is as follows:
(1) If the pellets of binder resin are directly mixed with the magnetic powder, the binder resin is separated from the magnetic powder upon input of the pellets of binder resin and magnetic powder in a kneader or during the kneading operation thereof, with a result that the content of the magnetic powder in the final pellets of the resin composition varies or the mixing/kneading of the binder resin and the magnetic powder becomes insufficient; and at the worst case, the mixing/kneading of the binder resin and the magnetic powder becomes impossible.
(2) If the resin composition is produced by using pellets of a resin binder, the accuracy in supplying a magnetic powder and the resin is degraded, to vary the extruding rate, which may obstruct the stable production of the resin composition.
(3) If the content of a magnetic powder in the pellets of the resin composition varies, the magnetic characteristic of a product molded from the resin composition also varies, resulting in not only degradation of quality in terms of magnetic characteristic but also lowering of the production yield and frequent occurrence of defectives.
The method of producing the resin composition characterized by pulverizing pellets of a binder resin once, however, has the following problems: namely, since the method requires a pulverizer for pulverizing pellets of a binder resin, the cost of equipment is raised, and since the method additionally requires the pulverizing step, the cost required for preparing a raw material is raised, thereby increasing the production cost of a resin magnet produced from the resin composition.
A first object of the present Invention is to provide a synthetic resin composition for a resin magnet, which is good in flowability upon melting, and which is capable of keeping the good flowability upon melting even if the content of a magnetic powder is increased and obtaining a high magnetic force of a molded product without lowering the moldability, and to provide a resin magnet molded from the synthetic resin composition.
A second object of the present invention is to provide a synthetic resin composition for a resin magnet, which is capable of preventing a reduction in dimensional accuracy due to contraction after molding as much as possible and also effectively preventing occurrence of expansion and camber due to an environmental change, thereby certainly obtaining a resin magnet excellent in dimensional accuracy by using the synthetic resin composition, and to provide a resin magnet molded from the synthetic resin composition.
A third object of the present invention is to provide a process of producing pellets of a synthetic resin composition for a resin magnet by mixing and dispersing a magnetic powder in a resin binder and extruding the mixture into pellets, which process is capable of reducing the production cost by omitting the step of pulverizing pellets of the resin binder.
From an examination to achieve the first object, the present inventor has found that, in the case of preparing a synthetic resin composition for a resin magnet by mixing and dispersing a magnetic powder in a resin binder composed of a thermoplastic resin, the addition of a polymerized fatty acid based polyamide elastomer to the resin binder makes it possible to effectively improve the flowability of the resin composition upon melting and keep a good flowability upon melting even if the content of the magnetic powder is increased for enhancing the magnetic force of a molded product, and hence to obtain a resin magnet having a high magnetic force without occurrence of lowering of moldability due to a reduction in flowability upon melting. The present inventor has thus accomplished the following first invention:
According to the first invention, there are provided a synthetic resin composition for a resin magnet, including a resin binder containing a thermoplastic resin as a main component and a polymerized fatty acid based polyamide elastomer, and a magnetic powder mixed and dispersed in said resin binder; and a resin magnet obtained by molding the synthetic resin composition into a desired shape.
To achieve the second object, the present inventor has examined principal causes of the reduction in dimensional accuracy, and obtained the following knowledge:
(1) If a crystalline resin such as polyamide or polyolefinic resin is used as the binder resin, the crystallinity of the binder resin rapidly proceeds when a molded product is cooled, which tends to enlarge the contraction and shrinkage of the molded product and to cause a camber of the molded product.
(2) If a hygroscopic resin such as polyamide is used as the binder resin, a molded product may be often expanded due to absorption of moisture in a service environment, which results in the environmental change in dimension.
The present inventor has further examined to solve these inconveniences, and found that the suitable addition of an ultra fine powder of an inorganic filler, having an average particle size of 1 xcexcm or less, to the binder resin in addition to a magnetic powder makes it possible to retard or obstruct the crystallinity of the binder resin when a molded product is cooled and thereby improve the dimensional accuracy of the molded product by suppressing the contraction thereof and also to suppress permeation of water molecules in the resin composition and thereby suppress the dimensional change due to moisture in a service environment, and hence to effectively solve the above-described inconveniences associated with the dimensional accuracy and to obtain a resin magnet which has a good dimensional accuracy and is capable of keeping such a good dimensional accuracy. The present inventor has thus accomplished the following second invention:
According to the second invention, there are provided a synthetic resin composition for a resin magnet, including a resin binder, a magnetic powder mixed and dispersed in said resin binder, and an ultra fine powder of an inorganic filler added to said resin binder, said powder having an average particle size of 1 xcexcg or less; and a resin magnet obtained by molding the synthetic resin composition into a is desired shape.
From an examination to achieve the third object, the present invention has found that, the adoption of a process of mixing pellets of a binder resin with a magnetic powder on a batch processing basis, supplying the mixture to an extruder at a relatively high speed by using a counting feeder, and extruding the mixture by the extruder makes it possible to produce pellets of a resin composition, in which the magnetic powder is mixed and dispersed in the binder resin desirably at a practical level, even if the step of pulverizing the pellets of the binder resin is omitted, and hence to produce a resin magnet such as a magnet roller having a sufficient performance at a low cost by injection-molding or extrusion-molding the resin composition thus produced into a desired shape. The present inventor has thus accomplished the following third invention:
According to the third invention, there is provided a process of producing pellets of a synthetic resin composition for a resin magnet, including the steps of mixing pellets of a binder resin with a magnetic powder on a batch processing basis, supplying the mixture to a kneading-extruder by means of a counting feeder, and extrusion-molding the mixture, to obtain pellets of a molding material for a resin magnet, in which the magnetic powder is mixed and dispersed in the binder resin.
The present inventor has examined to further improve the above-described process of producing a synthetic resin composition for a resin magnet and found that, when the extrusion speed upon extrusion-molding of the mixture of pellets of a binder resin and a magnetic powder is set at 30 kg/h or more per average volume of 100 mm3 of pellets, it is possible to further improve the uniform dispersibility of the magnetic powder, and when the pellets of resin composition thus obtained are uniformly mixed to be molded into a resin magnet, it is possible to obtain a resin magnet having a performance, particularly, a magnetic characteristic, comparable to or superior to that of a resin magnet obtained by using a resin composition prepared in accordance with the conventional process including the pulverizing step.
Accordingly, as one preferable aspect of the third invention, there are provided a process of producing a synthetic resin composition for a resin magnet, wherein the extrusion-speed by the kneading-extruder is set at 30 kg/h or more per average volume of 100 mm3 of pellets, and a process of producing a synthetic resin composition for a resin magnet, wherein the pellets of resin composition obtained by extrusion-molding are uniformly mixed to each other.